Light sources or illumination devices consisting of light emitting diodes (LEDs) are increasingly used for replacing conventional light sources such as incandescent lamps and fluorescent light sources. LEDs offer many advantages compared to conventional light sources, especially when it comes to light conversion efficiency. When LED-based light sources are used to replace traditional lighting systems, they are especially required to generate light that is perceived as white.
Color reproduction is typically measured using the color rendering index (CRI), which is calculated in Ra. The CRI is sometimes also referred to as color rendition index. The CRI is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source. Natural Daylight has a high CRI, where Ra is approximately 100. Incandescent bulbs too have a Ra close to 100, while fluorescent lighting is less accurate with an Ra of typically 70-90. Consequently, in order to achieve the desired “white” light in LED-based lighting applications, light sources with a high CRI are desirable. For LED lighting systems there are warm-white or neutral-white LED modules with a color rendering of about 80-90 readily available.
Conventional white LEDs typically use a blue light emitting element and a yellow wavelength converting material to convert part of the blue light to produce a mix of light that is perceived as white. An improved LED-based solution with respect to color rendition is disclosed in WO 2013/150470, which suggests a light emitting module comprising a blue light emitting element, at least one wavelength converting material arranged to receive light emitted by the blue light emitting element, and a “deep blue” light emitting element adapted to emit light having an emission peak in a second wavelength range of from 400 to 440 nm. This module provides white light of good color rendering with a “crisp white” effect, which is desirable for many applications, for example in retail or exhibition environments.
US2013114242 describes a solid state lighting device includes a solid state light emitter combined with a lumiphor to form a solid state light emitting component, at least one lumiphor spatially segregated from the light emitting component, and another lumiphor and/or solid state light emitter. The solid state light emitting component may include a blue shifted yellow component with a higher color temperature, but in combination with the other elements the aggregated emissions from the lighting device have a lower color temperature. Multiple white or near-white components may be provided and arranged to stimulate one or more lumiphors spatially segregated therefrom.
US2013221866 describes an illuminating device comprising a plurality of semiconductor light-emitting devices differing in emission color and employing a semiconductor light-emitting element and a phosphor, wherein outputted light is stably combined, separation of light is inhibited, and color tone is variable, and which devices emit light outward on the basis of an emission from the semiconductor light-emitting element and from the phosphor which is excited by emission from the semiconductor light-emitting element to fluoresce or on the basis of emission from the phosphor which is so excited to fluoresce, the deviation duv of which from a blackbody radiation locus being within a range of −0.02≦duv≦0.02, in the uv chromaticity diagram according to UCS (u,v) color system (CIE 1960); and outputted lights from the light-emitting part in which the plurality of kinds of the semiconductor light-emitting devices are integrated and arranged, are mixed together and emitted outward.
WO2007114614 describes a light emitting device comprising a first light emitting portion that emits white light at a color temperature of 6000K or more and a second light emitting portion that emits white light at a color temperature of 3000K or less, which include light emitting diode chips and phosphors and are independently driven. WO2007114614 indicates that a light emitting device can be diversely applied in a desired atmosphere and use by realizing white light with different light spectrums and color temperatures. WO2007114614 indicates the effect on health by adjusting the wavelength of light or the color temperature according to the circadian rhythm of humans.
US2012256560 describes a light emitting device package includes a package body; a first light emitting device mounted on the package body and emitting light of a particular color; a second light emitting device mounted on the package body to be adjacent to the first light emitting device, adjusting an amount of light according to a current value applied thereto to thereby control a color temperature, and emitting orange light; and a resin part sealing the first and second light emitting devices and containing at least one or more types of phosphors.